1. Field of the Invention
The present invention relates to a method employing measurement of magnitude and direction of the bending moments near a drill bit to estimate formation dip at an interface.
2. Description of the Prior Art
The dip of a formation is useful to geologists and reservoir engineers in defining the type, size and the profile of a reservoir. Further, this information is useful for explaining directional drilling tendencies, for correlating lithology, and for detecting faults in a formation. The angle (magnitude) and direction of the formation dip is presently measured by passing a hard wired, wireline device through a completed hole. Although measurements made by this manner provide useful information, they are of no help to the drilling engineer during the drilling operation.
Because the dip of a formation can affect the side forces acting on a bit while drilling, knowledge of the formation dip would be most useful to the drilling engineer particularly, when he is attempting directional drilling. The present invention provides a method which is useful for predicting or determining the magnitude (angle) and direction of formation dip by measuring the magnitude and direction of bending moments on the bit while the drilling operation continues. Measurements of the bending moment are made in two orthogonal planes providing both magnitude and direction for the bending moments. This is accomplished by monitoring the direction of the two orthogonal planes by using oriented magnetometer measurements.
U.S. Pat. No. 4,445,578 to Millheim discloses are apparatus and method for providing measurement of the side force on a drill bit during drilling, thus permitting corrective action to be taken immediately in the drilling operation. The Millheim system includes means to detect the side thrust or force on a bit and the force on the deflection means of a downhole motor. This system provides for measuring the magnitude of the force on a downhole stabilizer. While Millheim discloses means for measuring various forces acting near the drill bit and correcting the drilling parameters in response thereto, he does not disclose or suggest any way in which these measurements can be used to make a determination of the formation dip. The side forces at the bit or at a sub are measured by using multiple strain gauges or load cells and transmitting the measurements back to the surface. The sampling rate is limited by the transmission rate. The measured forces are then used to determine the directional tendencies of the hole. The orientation of the side forces are not measured, but periodic surveys of the hole are made to determine its direction during rotary drilling.
U.S. Pat. No. 4,324,297 to Denison discloses a method and apparatus for measuring the weight on bit, the bending stress near the bit, and the orientation of these stresses. These measurements are sent to the surface by wire line telemetry or other high data rate transmission means including mud pulse telemetry. The data is processed at the surface to compare the measured side forces with a drilling model for controlling the directional tendencies by adjusting weight on bit. This patent teaches the use of oriented bending moments for directional control. In order to effectively implement the teachings of this patent it is necessary to have a high data rate telemetry system. However, this patent does not mention anything about measuring the formation dip or how interaction with a formation face will affect the steering or the possibility of utilizing downhole processing to avoid transmission rate limitations and associated problems.